Текст B. Concrete is subject to dimensional changes due to temperature variations, initial shrinkage and moisture movements among others

Concrete is subject to dimensional changes due to temperature variations, initial shrinkage and moisture movements among others. Concrete has low tensile strength and the dimensional changes may cause cracking of the concrete. Measures which prevent cracking are necessary in the design of concrete structures but may not always be completely successful.

An elastic material under load is subject to a deformation which causes an elastic strain. Some materials, including concrete, continue to change although there is no increase in load. This deformation causes a further strain, the creep strain, which for concrete, over time, is usually greater than the elastic strain; the creep strain always increases with time and finally reaches a limit. Creep is higher at higher temperatures and is about double at 70°C (158°F) that at 20°C (68°F). The effects of shrinkage can be reduced if construction joints are provided.

Concrete has a relatively high value of elastic modulus. For example, a concrete column of common proportions has only elastic deflections under load. Concrete may also be useful as a tie when it is required to limit the extension; the tie would normally be prestressed. Long term loads cause further slow deformations by creep and this effectively reduces the value of the elastic modulus of the concrete.

Notes:

elastic strain упругая деформация

measure n. мера

Ответьте на вопросы к тексту.

1. What causes dimensional changes of concrete?

2. What does a deformation cause in an elastic material?

3. What does elastic strain result in?

4. How does creep depend on temperature?

5. How can the effects of shrinkage be reduced?

6. Does concrete have a high or low value of elastic modulus?

7. When may concrete be useful as a tie?

8. What reduces the value of the elastic modulus of the concrete?

Date: 2016-02-19; view: 446; Нарушение авторских прав